RATIONALE: Monoclonal antibodies, such as rituximab, can block cancer growth in different ways. Some block the ability of cancer cells to grow and spread. Others find cancer cells and help kill them or carry cancer-killing substances to them. Drugs used in chemotherapy, such as pentostatin and cyclophosphamide, work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Lenalidomide may stop the growth of cancer cells by blocking some of the enzymes needed for cell growth. Giving rituximab together with combination chemotherapy and lenalidomide may kill more cancer cells.